1. Field of the Invention
The present invention relates to a light-emitting element drive circuit system and an electronic device, and more particularly to a light-emitting element drive circuit system for gradually changing luminance or the like of light-emitting elements, and an electronic device including such a light-emitting element drive circuit system.
2. Description of the Related Art
In recent years, light-emitting element drive circuit systems are provided in various electronic devices such as mobile phones. By causing the light-emitting elements to emit light (or to be turned ON), characters and patterns are displayed on LCD and other screens. In doing so, there are cases in which luminance and the like of light-emitting elements are gradually changed. In other words, the light-emitting elements are caused to emit light that changes in gradation.
As a related art of the present invention, JP 2005-11895 A discloses an LED drive circuit for driving an LED using a battery. The LED drive circuit includes a constant current circuit inserted on the anode side or the cathode side of an LED for controlling the current flowing through the LED to have a predetermined target value, and a resister connected on the cathode side of the LED and downstream of the constant current circuit. The LED drive circuit further includes a battery in which the voltage varies within a range including a predetermined voltage value and in accordance with the remaining available capacity, wherein the predetermined voltage value is a sum of a forward voltage decrease in the LED, a drive voltage in the constant current circuit when achieving the predetermined target value, and voltages at the two ends of the resistor when achieving the predetermined target value. The LED drive circuit also includes a booster circuit connected between the battery and the LED. When a switch provided inside the booster circuit is turned ON, the booster circuit boosts up the battery voltage to a magnitude greater than or equal to the predetermined voltage and outputs the boosted voltage, and, when the switch is turned OFF, the booster circuit outputs the battery voltage without changing. Further, the LED drive circuit includes a control circuit connected to the constant current circuit. The control circuit detects the magnitude relationship between the battery voltage and the predetermined voltage, and, only when the battery voltage becomes lower than the predetermined voltage, the control circuit turns on the switch inside the booster circuit.
Among light-emitting element drive circuit systems as shown in FIG. 7, there are drive circuit systems which serve to change the value of a light-emitting element drive current in order to cause a light-emitting element to emit light that changes in gradation (this current output from a gradation current circuit 90 is referred to as “a gradation current”). For example, as shown in FIG. 7, a reference current (Iref) output from a reference current circuit 20 is subjected to calculations in the gradation current circuit 90 and amplified in an LED driver circuit 60, so that a light-emitting element drive current as shown in FIG. 8 can be made to flow.
More specifically, in the gradation current circuit 90, calculation is performed according to the following arithmetic expression: Igra (output current from the gradation current circuit 90)=Agra*Iref*m/n, where Agra is an arbitrary constant, n is a predefined natural number, and m is 0, 1, 2, . . . n (transition period T is divided into n sections). Subsequently, in the LED driver circuit 60, amplification is performed according to the arithmetic expression ILED=ALED*Igra, where ALED is an arbitrary constant. As a result, over the duration of a predefined transition period T from time a1 to time a2, the current is varied linearly from current value 0 to current value ILED1. From time a2 to time a3, current value ILED1 is maintained. Furthermore, during the period from time a3 to time a4, the current is output while being varied linearly from current value ILED1 to current value 0. By performing a similar procedure, the light-emitting element drive current having the current characteristic as shown in FIG. 8 is also output during the period from time a4 to time a7.
In a case where a light-emitting element 8 is driven by the above-described light-emitting element drive circuit system, the current has a slope and is varied linearly during the periods from time a1 to time a2, from time a3 to time a4, from time a4 to time a5, and from time a6 to time a7 shown in FIG. 8. Accordingly, during these periods, the light-emitting element 8 emits light that changes in gradation; i.e., performs gradation emission. However, according to the light-emitting element drive circuit system shown in FIG. 7, gradation emission of the light-emitting element 8 can only be performed when the drive current value is caused to change from current value 0 to current value ILED1 (or current value ILED2), and from current value ILED1 (or current value ILED2) to current value 0. As it is impossible to perform gradation emission of the light-emitting element 8 when causing the drive current value to change from a first current value not equal to zero to a second current value that is not equal to both zero and the first current value, gradation emission may only be performed limitedly.